The present invention relates in general to techniques and devices for obtaining the surface characteristics of semiconductor materials, and in particular to a new and useful method and apparatus for the non-destructive surface acoustic wave study of semiconductor surface properties.
A non-destructive surface acoustic wave (SAW) technique has been used to study semiconductor surface properties. Surface acoustic waves are generated by applying an rf pulse to interdigital transducers made on the surface of a piezoelectric material (LiNbO.sub.3) by evaporation and photolithography. Because LiNbO.sub.3 is piezoelectric, the acoustic wave is accompanied by an electric field with a component perpendicular to the surface of the LiNbO.sub.3 substrate. This component exists outside the LiNbO.sub.3 to a distance of about an acoustic wavelength (about 31.6 .mu.m for f=110 MHz). The SAW measurement technique relies on the nonlinear interaction between this probing electric field and the free carriers of the semiconductor under study which is placed above the piezoelectric substrate. The penetration depth of the electric field inside the semiconductor is on the order of the semiconductor extrinsic Debye length or the acoustic wavelength, whichever is shorter. As a result of this nonlinear interaction, a transverse acoustoelectric voltage (TAV) develops across the semiconductor.
The TAV amplitude dependence on the electronic properties of the semiconductor surface is proportional to the conductivity difference between the electrons and holes, i.e., ##EQU1## where: n,p=free electron and hole concentrations within the interaction depth;
.mu..sub.n, .mu..sub.p =electron and hole mobilities; PA1 w=SAW angular frequency; PA1 e.sub.p =permitivity of the piezoelectric substrate; PA1 e.sub.s =permitivity of the semiconductor under study; PA1 P. Das. M. E. Montamedi, and R. T. Webster, Appl. Phys. Lett. 27,120 (1975); PA1 P. Das, M. E. Montamedi, H. Gilboa, and R. T. Webster, J. Vac. Sci. Technol 13, 9481 (1976); PA1 H. Gilboa and P. Das; "Nondestructive Evaluation of Electrical Properties of Semiconductors Using SAW," Technical Report MA-ONR-15, RPI, Troy, N.Y., July (1977); PA1 P. Das, M. K. Roy, R. T. Webster, and K. Varahramyan, IEEE Ultrasonic Symposium Proceedings, p. 278, September (1979); PA1 H. Estrada-Vazquez, R. T. Webster, and P. Das, J. Apply. Phys.50, 4942 (1979); PA1 B. Davari and P. Das, J. Appl. Phys. 53, 3668(1982); PA1 B. Davari and P. Das, Appl. Phys. Lett. 40,807(1982); PA1 P. Das, R. T. Webster, and B. Davari, Appl. Phys. Lett. 34,307(1979); S. M. Sze, Physics of Semiconductor Devices, 1st ed. (Wiley, N.Y., 1969), p.425; PA1 R. T. Webster, H. Estrada-Vazquez, P. Das, and R. Bharat, Solid State Electron, 22, 541 (1979); PA1 Haim Gilboa and Pankaj K. Das, IEEE Trans. Electron Devices ED-27, 461 (1980); PA1 S. K. Ghandi, The Theory and Practice of Microelectronics 1st ed. (Wiley, N.Y., 1968), p.418; PA1 A. Many, Y. Goldstein, and N. B. Grover, Semiconductor Surfaces, 2nd ed. (North-Holland, N.Y. 1971), p.149; PA1 K. Varahramyan, R. T. Webster and P. Das, J. Appl. Phys., 51,1234 (1980); PA1 M. E. Motamedi and P. Das, J. Appl. Phys., 48, 2135 (1977). PA1 W. Van Gelder and E. H. Nicollian, J. Electrochem. Soc. 118,138 (1971); PA1 J. Verjans and J. Van Overstraeten, Solid-State Electron 18,911 (1975); PA1 G. L. Miller, IEEE Trans. Electron Devices ED-19,1103(1972). PA1 A. Goetzberger and E. H. Nicallian, J. Appl. Phys., 38,4582 (1967); PA1 D. P. Kennedy, P. C. Murley and W. Kleinfeld, IBM J. Res. Develop. 12,399 (1968) PA1 D. P. Kennedy and R. R. O'Brien, IBM J. Res. Develop. 13, 212(1969); PA1 E. H. Nicollian, M. H. Hanes, and J. R. Brown, IEEE Trans. Electron Dev., ED-20, 280 (1973); PA1 W. C. Johnson and P. T. Panousis, IEEE Trans. Electron Div., ED-18, 965 (1971); PA1 G. Baccarani, M. Rudan and G. Sqadina, Solid-State Electron, 23, 65 (1980); PA1 R. A. Moline, J. Appl. Phys. 42, 3553 (1971); and PA1 B. Davari and P. Das, IEEE ULTRASONIC SYMPOSIUM PROCEEDINGS, p.379, (1982).
and ##EQU2## where: ##EQU3## D.sub.n,D.sub.p =electron and hole diffusivity; and ##EQU4##
The constant V.sub.o in equation (1) is related to temperature, piezoelectric coupling coefficient, frequency, and the acoustic power. Surface properties of the semiconductor can be studied by varying the surface conductivity while monitoring the TAV signal. Surface conductivity can be varied by photons, heating or cooling, and dc voltage applied across the semiconductor.
The photon excitation has been used in the form of one- and two-beam TAV spectroscopy to determine GaAs and CdS surface band structures. The application of dc voltage to study the surface properties of silicon has also been used.
The following is pertinent to the foregoing analysis and this application:
Impurity atom profiling of semiconductors has also been performed by different capacitance-voltage (C-V) techniques such as pulsed C-V, the second harmonic method and the feedback technique. In these methods the measured physical quantity is the width of the approximated depletion layer (W). The majority carrier concentration is evaluated by analyzing the dependency of W on a variable bias voltage as applied across an MOS, p-n or Schottky barrier junction. The impurity concentration profile is then obtained from the majority carrier concentration profile.
References which are pertinent to these techniques are: